Mesoscopic physics: From low-energy nuclear [1] to relativistic [2] high-energy analogies

Transcription

1 Mesoscopic physics: From low-energy nuclear [1] to relativistic [2] high-energy analogies Constantine Yannouleas and Uzi Landman School of Physics, Georgia Institute of Technology [1] Ch. 4 in Metal Clusters, edited by W. Ekardt (John-Wiley, New York, 1999) pp ; Rep. Prog. Phys. 70 (2007) pp [2] PRB 89, (2014); PRB 87, (2013) NMP14, 5-9 May 2014 Supported by the U.S. DOE (FG05-86ER45234)

2 Mesoscopics: The area of condensed-matter physics that covers the transition regime between macroscopic objects and the microscopic, atomic world. TU Delft course Finite-size condensed-matter nanosystems (small systems and transition to the bulk) Nuclear analogies (nonrelativistic electrons/ Schrödinger equation): (3D) metal clusters, metal grains,fullerenes; (2D) quantum billiards, quantum dots; quantum islands; (1D) quantum-point contacts, nanowires, quantum rings, interferometers Particle-physics analogies (relativistic electrons/ Dirac equation): Graphene-based nanosystems: (2D) graphene quantum dots; (1D) uniform and segmented graphene nanoribbons (junctions), graphene polygonal rings

3 FIRST PART Some examples (among many, e.g., random matrix theory) of nuclear analogies (from personal experience) In this talk: Emphasis on broader qualitative aspects and not on mathematical theoretical formulation Collaborators: Uzi Landman, Igor Romanovsky, Yuesong Li, Ying Li, Leslie Baksmaty, R.N. Barnett

4 Three (among others) major nuclear aspects: Electronic shells/deformation/fission (via Strutinsky/ Shell correction approach) in metal clusters [see, e.g., Yannouleas, Landman, Barnett, in Metal Clusters, edited by W. Ekardt, John-Wiley, 1999] Surface plasmons/giant resonances (via matrix RPA/LDA) in metal clusters [see, e.g., Yannouleas, Broglia, Brack, Bortignon, PRL 63, 255 (1989)] Strongly correlated states (Quantum crystals/wigner molecules/dissociation) in 2D semiconductor quantum dots and ultracold bosonic traps via symmetry breaking/symmetry restoration in conjunction with exact diagonalization (full CI) [see, e.g., Yannouleas, Landman, Rep. Prog. Phys. 70, 2067 (2007)]

5 Electronic shells/ magic numbers/ deformation/ fission in metal clusters Surface plasmons/giant resonances in metal clusters The physics of free nonrelativistic electrons confined in a central potential, like atomic nuclei (conservation of symmetries/ independent particle model/ delocalized electrons) Strongly correlated states (Quantum crystals/wigner molecules/dissociation) in 2D semiconductor quantum dots No central potential/ electron localization (relative to each other) due to strong Coulomb repulsion/ mean-filed with broken symmetries

6 Devices Lateral QD (Ottawa) Vertical QD (Delft) Electrostatic confinement Lateral QD Molecule (Delft)

7 Control parameters Neutral bosons

8 Circular external confinement N=19e Wigner molecule in a 2D circular QD. Electron density (ED) from Unrestricted Hartree-Fock (UHF). Symmetry breaking (localized orbitals). Concentric polygonal rings N=6e Concentric rings: (0,6) left, (1,5) right Y&L, PRL 82, 5325 (1999) Restoration of symmetry Concentric rings: (1,6,12) Y&L, PRB 68, (2003) Exact electron densities are circular! No symmetries are broken! (N, small, large?) Quantum crystal

9 Rotating Boson Molecules (Circular trap) Ground states: Energy, angular momentum and probability densities. R W =10 Probability densities ED Rotating Frame Magnetic Field ED The hidden crystalline structure in the projected function can be revealed through the use of conditional probability density (CPD). CPD r 0

10 Three electron anisotropic QD Method: Exact Diagonalization (EXD) Anisotropic confinement Electron Density (ED) ED ED CPD (spin resolved) Conditional Probability Distribution (CPD) CPD CPD CPD Yuesong Li, Y&L, Phys. Rev. B 76, (2007) EXD wf ~ > - > Entangled three-qubit W-states

11 WAVE-FUNCTION BASED APPROACHES TWO-STEP METHOD EXACT DIAGONALIZATION (Full Configuration Interaction) When possible (small N): High numerical accuracy Yannouleas and Landman, Rep. Prog. Phys. 70, 2067 (2007) Physics less transparent compared to THE TWO-STEP Pair correlation functions, CPDs

12 RESOLUTION OF SYMMETRY DILEMMA: RESTORATION OF BROKEN SYMMETRY BEYOND MEAN FIELD (Projection)! Per-Olov Lowdin.. (Chemistry - Spin) R.E. Peierls and J. Yoccoz (Nuclear Physics L, rotations) Ch. 11 in the book by P. Ring and P. Schuck Note: Example in 2D Yannouleas, Landman, Rep. Prog. Phys. 70, 2067 (2007)

13 1 Weizmann Institute of Science, Israel 2 Niels Bohr Institute, Denmark 5 Physics Department, Cornell University, Ithaca, New York

14 SECOND PART Some examples of high-energy particle-physics analogies (graphene based nanosystems) I. Romanovsky, C. Yannouleas, and U. Landman, PRB 89, (2014) PRB 87, (2013)

15 2D Graphene: honeycomb lattice c v_f Graphene Nanosystems Armchair or Zigzag edge terminations Massless Dirac-Weyl fermion Graphene nanoribbons Graphene quantum dots Open a gap D? M v_f^2 = D Graphene nanorings

16 Uniform Armchair Nanoribbons D, M D, M TB N=3m (Class I) Semiconductor Massive Dirac N=3m+1 (Class II) Semiconductor k /3a x N=3m+2 (Class III) Metallic N x (tight binding)

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18 Tight-Binding (TB) 2.7 ev

19 Two atoms in a unit cell/ Two sublattices A and B Tight-Binding (TB)

20 N=16 (Class II) N=15 (Class I) Aharonov-Bohm spectra Magnetic field B Hexagonal Armchair Rings with semiconducting arms Single-particle TB spectra Magnetic flux (magnetic field B)

21 1D Generalized Dirac equation a and b: any two of the three 2x2 Pauli matrices electrostatic potential (Lorentz vector potential) scalar (Higgs) field / position-dependent mass m(x) (Lorentz scalar potential) Question: Confinement of a relativistic fermion? Problem with V(x): Klein tunneling m(x) can confine relativistic particles

22 1D Generalized Dirac equation a and b: any two of the three 2x2 Pauli matrices electrostatic potential scalar (Higgs) field / position-dependent mass m(x) Dirac-Kronig-Penney Superlattice a single side/ 3 regions m2 m1 m x Transfer matrix method

23 N=16 (Class II) N=15 (Class I) Spectra/ Armchair Rings with semiconducting arms Yellow: Mass > 0 Red: Mass < 0 Magnetic flux (magnetic field B)

24 Jackiw-Rebbi, PRD 13, 3398 (1976) kink soliton/ zero-energy fermionic soliton kink soliton D1 zero-energy fermionic soliton (Dirac eq.) D D topological insulator Topological invariants (Chern numbers): negative mass 1 (nontrivial) positive mass 0 (trivial)

25 e/6 fractional charge Densities for a state in the forbidden band

26 Mixed Metallic-semiconductor N=17 (Class III) / N=15 (Class I) e/2 fractional charge

27 Conclusions Full circle 1) Instead of usual quantum-size confinement effects (case of clusters/ analogies with nuclear physics), the spectra and wave functions of quasi-1d graphene nanostructures are sensitive to the topology of the lattice configuration (edges, shape, corners) of the system. 2) The topology is captured by general, position-dependent scalar fields (variable masses, including alternating +/- masses) in the relativistic Dirac equation. 3) The topology generates rich analogies with 1D quantum-field theories, e.g., localized fermionic solitons with fractional charges associated with the Jackiw-Rebbi model [PRD 13, 3398 (1976)] 4) Semiconducting hexagonal rings behave as 1D topological insulators with states well isolated from the environment (zero-energy states within the gap with charge accumulation at the corners).